stats regression test's wait_for_stats() must check timestamp too.

stats regression test's wait_for_stats() must check timestamp too.

pg_stat_get_snapshot_timestamp() returns the timestamp seen in the "global"
stats file.  Because pgstat_write_statsfiles() writes per-DB stats files
before the global file (or at least before renaming it into place), there
is a window where the test backend can see all the stats updates that
wait_for_stats() was checking for (all of which come from the per-DB file)
but also see the same global stats file it had seen at the start of the
test script.  This results in a failure in only the "snapshot_newer" query,
as reported by a couple of buildfarm members recently.

I suspect that this ought to be back-patched.  Commit 4e37b3e15 has
evidently increased the probability of this window getting hit, but
it's not apparent why it could not have been hit before.  I'll refrain
for the moment though.